Natural fracture systems in Mowry Shale outcrops, Bighorn Basin, Wyoming

Abstract

<p>The Upper Cretaceous Mowry Shale is a siliceous, organic-rich unit that is considered to be one of several source rocks in the Bighorn Basin, north-central Wyoming. Rocks in the Bighorn Basin were deformed in compression during the Late Cretaceous to early Eocene (?) Laramide orogeny. Recently, the potential for significant hydrocarbon production from the Mowry Shale has been reported. Therefore, the goal of this project is to investigate local and regional variability of fracture development within the Mowry and to consider the implications of its natural fracture system for hydrocarbon production in the Bighorn Basin.</p> <p>Outcrop studies were conducted in areas of strong folding (Alkali Anticline and north nose of Sheep Mountain Anticline, northwest of Greybull, WY) and areas with nearly flat-lying beds (Greybull Platform, east of Greybull) to characterize the orientation, length, spacing, and intersection relations of various fracture sets.</p> <p>Six major fractures sets were identified in all study areas with average strikes of 15°, 50°, 90°, 110°, 140°, and 165°. Field fracture data along with petrographic study of micro-fractures suggest that all the fracture sets are extensional. The 50° set formed parallel to the maximum Laramide compressional stress. It occurs pervasively through the strongly folded regions and in parts of the Greybull Platform Laramide that were impacted by Laramide deformation. The 140° set is orientated from parallel to subparallel to the fold hinge of the Laramide structures and is probably related to extension during bending of layering. This set appears as local fracture swarms in most outcrops in the gently folded Greybull Platform. The 110° set likely formed pre-Laramide, and occurs in tightly folded areas, as well in brittle strata. The 15° and 165° sets are possible pre-Laramide, and are best developed in the Greybull Platform. The 90° set may have formed during the Sevier orogeny, and occurs in all study areas, but is best developed on the Greybull Platform. Surface fracture orientations that are potentially present in the subsurface of the Bighorn Basin include the 15°, 90°, 140° (fracture swarms) and 165° sets.</p>